I would never claim to argue a percentage of the aikidoka populace, just a percent of the practitioners I have experience in dealing with. Nor would I expect anyone else to make such a claim. Taking that as a hook, the average five year practitioner from aikido compared to the average five year practitioner from Wing Chun is less likely to handle a salvo of incoming attacks or follow a standard attack, counter, counter-counter prgression. It is the counter-counter that I have seen be the problem. Also, is it not possible for a mixture of both to occur: the attack landing and the defense working, each on a relative scale? It is cases such as these that I see most often in close quarter combat. Hence an explanation for the statement above, understanding that the training elements of Wing Chun deal more directly with such conditions.
A failed defense= not redering the opponent "out of range, active threat" And i have seen this occur numerous times among very well trained aikidoka who engage Wing Chun and/or Uechi ryu practitioners of similar caliber. Not to say that such is a defficiency, but a description of elements. Similar descriptions can be made given certain conditions of any art.
Cross training is the best answer if it is of that much concern.
The ultimate answer being that if one is of greater skill than the opponent all of this debate is a mute point. "Lengthen your line."